The activation of various types of surfaces with solutions or colloids containing precious metals such as palladium, gold and silver has been known for quite some time. The most rudimentary activators consisted essentially of precious metals, most predominantly palladium, in aqueous solutions. A typical activation composition of this type might consist of an aqueous solution of palladium chloride, sodium chloride, and hydrochloric acid. The surface to be activated was merely dipped into this activating composition, rinsed, then electrolessly plated. These type activation compositions are still in use today.
The next generation of activator systems consisted of two sequential steps. The first step consisted of a solution of a Group IV metal ion reducing agent, such as stannous ion, which solution applied a film of this reducing metal ion to the surface to be activated. The second step consisted of an aqueous solution of a precious metal, most predominantly. Thus, the initial Group IV metal ion reducing agent reduced precious metal ions to their zero valence state on the surface. The surface was then electrolessly plated.
More recent advances in activation involve the combination of the two-step activating systems into a single step. These single step activators consist mainly of tin-palladium colloids. A recent improvement to these generic tin-palladium colloidal activators is described in U.S. Pat. No. 4,863,758, the teachings of which are incorporated herein by reference in their entirety.
Regardless of the exact nature of these prior art activators, they all share at least one characteristic in common in that they all activate a variety of surfaces indiscriminately. This is particularly true in the area of printed circuit boards. These activators are used extensively to activate both the metallic and the plastic (usually epoxy-glass) surfaces of printed circuit boards for subsequent electroless metal plating. These activator compositions activate both types of surfaces indiscriminately, however, in many cases this indiscriminate nature of activation causes difficulty and/or inefficiency in the manufacture of printed circuit boards. In particular, in some cases, it is desirable to activate the metallic surfaces (usually copper or nickel) on the printed circuit board, but not the plastic surfaces, thus enabling selective plating on the metallic surfaces as opposed to the plastic surfaces.
This is particularly true in the plating of exposed metallic areas of the circuits after a majority of the board surface has been covered by solder mask (for a discussion of soldermasks please see U.S. Pat. No. 5,296,334, Castaldi et al., the teachings of which are incorporated herein by reference in their entirety). Generally, at this point, the manufacture of the circuit board is almost complete. The circuits and through-holes, if any, have been defined and plated, and the board itself has been coated with a permanent layer of solder mask. The solder mask generally covers all areas of the board, except for the holes, vias, pads, tabs, lands, and other areas of connection (collectively "areas of connection"). These areas of connection are usually copper or nickel areas which are exposed in order for subsequent connections of the components to be made thereto. These copper areas of connection require further treatment to protect or enhance their solderability or their ability to make other types of connection in a reliable manner.
Several methods of treatment of these areas have been proposed. Some proposals involve coating or pre-coating these areas with solder as described in U.S. Pat. No. 5,160,579, the teachings of which are incorporated by reference herein in their entirety. Other proposals involve the treatment of these copper surfaces with various organic pre-fluxes or anti-corrosion agents as described in U.S. Pat. No. 5,173,130, the teachings of which are incorporated by reference herein in their entirety. By far, however, the most popular way of treating these areas of connection is by overplating the copper with a barrier layer usually consisting of a metal such as nickel, followed by a second overplating with a precious metal such as gold, palladium or rhodium ("nickel-gold process" generally). These processes are aptly explained in U.S. Pat. Nos. 4,940,181 and 5,235,139, the teachings of which are both incorporated herein by reference in their entirety.
One of the difficulties with nickel-gold processes is that the copper surfaces are not catalytic to the electroless nickel-phosphorous baths which are used. U.S. Pat. No. 5,235,139, the teachings of which are incorporated herein by reference in their entirety, teaches the use of a nickel-boron coating prior to the nickel-phosphorous coating in an attempt to make the surface catalytic to the nickel-phosphorous bath. This approach is widely used but improvements are still desirable since incomplete coverage is still sporadically and unpredictably observed in the absence of a separate activation step.
It should be pointed out that various palladium or precious metal activators will activate copper surfaces for the reception of electroless nickel-phosphorous, but they will also indiscriminately activate other areas of the board on which plating is not desirable in this application. The current invention then addresses this concern and proposes an activator which will selectively activate the copper surfaces and thereby minimize or eliminate extraneous plating.